Recently, various catalysts and methods have been proposed for thermal decomposition of an organic substance such as waste plastic. For example, a method has been proposed in which granules of titanium oxide are used as a catalyst for thermal decomposition of waste plastic. The catalyst is prepared by sintering fine powder of titanium oxide to be a particle diameter of 2 mm or more and processing the granules so as to have an appropriate particle size distribution. According to the method, the granules of titanium oxide and waste plastic strips are heated with stirring in a reaction vessel while a carrier gas is flowed into the vessel, thereby the waste plastic is thermally decomposed, gasified, and exhausted out of the vessel (Patent Literature 1).
However, the method has some problems. That is, the granules of titanium oxide obtained as above are readily worn and pulverized because they have insufficient hardness, and hence, during the thermal decomposition of waste plastic as described above, the pulverized titanium oxide is exhausted out of the vessel accompanied with the gas generated by thermal decomposition of waste plastic. Consequently, efficiency of the thermal decomposition is decreased with the progress of time. Moreover, as the granules of titanium oxide are pulverized, the particle size distribution is changed, and the thermal decomposition efficiency of waste plastic is also decreased.
When a filter is attached to a pipeline for exhausting thermal decomposition gas from the reaction vessel, pulverized titanium oxide can be collected (Patent Literature 2). However, the filter is readily clogged, and thus the thermal decomposition efficiency of waste plastic is decreased with time. On the other hand, even if granules of titanium oxide having large particle diameter are used in order to avoid the pulverization of titanium oxide, the thermal decomposition efficiency of waste plastic becomes poor.
In order to solve such problems, a method in which titanium oxide sol is dried, calcined, crushed, and then subjected to edge processing for satisfying both wear resistance and decomposition performance has been developed (Patent Literature 3). According to the method, the problems described above can be solved, but there is a problem of treatment of fine powder generated by the edge processing. Namely, there is a problem that the fine powder generated is difficult to be recycled.    Patent Literature 1: JP 2005-306697A    Patent Literature 2: JP 2002-363337A    Patent Literature 3: JP 2005-307007A